The developing trend of the DC/DC converter is just like that of the most of the power supply products, that is—high efficiency. The resonant DC/DC converter is easier to realize the high efficiency due to its feature of soft-switching. However, there are still certain existing drawbacks regarding the resonant DC/DC converter, e.g., the high ac current of the output filter of the series resonant DC/DC converter resulting in the high power loss and the large volume of the output filter.
FIGS. 1(a)-1(d) are schematic circuit diagrams of several kinds of resonant DC/DC converter circuit. FIG. 1(a) shows a series resonant converter which includes a DC power source providing an input voltage Vin, a first and a second switches S1-S2, a resonant capacitor Cs and an output capacitor Co wherein the output voltage Vo can be gotten on it, an inductor Ls, a transformer T, diodes D1-D2 and load Ro. The differences between FIG. 1(b) and FIG. 1(a) are that a capacitor Cp is parallel-connected to the primary side of the transformer T; the resonant capacitor Cs is omitted; and an inductor Lr among the secondary side of the transformer T, the diode D1 and the output capacitor Co is added. FIG. 1(c) shows a parallel resonant converter, e.g. an LCC resonant converter and the difference between FIG. 1(c) and FIG. 1(b) is that the resonant capacitor Cs connected in series with the inductor Ls is added. The difference between FIG. 1(d) and FIG. 1(a) is that a magnetizing inductor Lm is parallel-connected to the primary side of the transformer T. Taking the example of the LLC series resonant DC/DC converter as shown in FIG. 1(d), the operating waveforms are shown in FIG. 2. S1 and S2 indicates the driving signals of the switches S1-S2 respectively; is and im are currents flowing through the resonant inductor Ls and the magnetizing inductor Lm respectively; im has the values of Im and −Im respectively when switches S1 and S2 are turned off; Vds 1 is the voltage between the drain and the source of the switch S1; iD1 and iD2 are current waveforms of the output rectifying diodes D1 and D2; Io is the output current of the converter; iD1+iD2−Io is the current flowing through the output filter (output capacitor) Co; Vcs is the voltage across capacitor Cs; and all the waveforms in FIG. 2 operate in six intervals (t0-t1, t1-t2, . . . and t5-t6) per period, and iterate from the seventh interval (t6=t0). And since iD1 and iD2 have larger ripples, the ac current value of the output filter (output capacitor) Co is large which results in large size of Co and high power loss of the converter.
To decrease the ac current of the output filter (output capacitor) Co, the interleaved method is always used to control the resonant converters, wherein the interleaved method means that at least two converters operate at substantially the same frequency and with some phase φ (0°<φ<360°) shifted between them. However, some problems still exist due to the characteristics of the resonant converters.
When the interleaved control method is adopted, the resonant converters operating at the substantially the same frequency and with some phase shifted between them are always connected in parallel at a common output filter and their input terminals are all connected together. Thus the ac current of the output filter (e.g. the output capacitor) Co is cancelled and the effect of the cancellation is the function of the shifted phase φ so that the size of the output filter (output capacitor) Co is decreased. The interleaved control method is widely used in PWM converters since they operate at constant frequency and could regulate the output voltage and the current through changing the duty ratio such that the current balance between the interleaved PWM converters is easy to be realized. While in a resonant converter, the regulations of the output voltage and the current are realized through changing the frequency. If the resonant converters are forced to operate in the same frequency, the current balance between the interleaved resonant converters is hard to be realized due to their different characteristics. On the contrary, if each converter regulates the voltage and the current on its own so as to realize the current balance, they could not operate at the same switching frequency so as to lose the advantage of the controlling method for the interleaved and parallel-connected configuration.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a parallel-connected resonant DC/DC converter circuit and a controlling method thereof.